1. Field of the Invention
This invention relates to improvements in a stabilized d.c. high voltage power source, and more particularly to improvements in a system for measuring a load current of such power source.
2. Description of the Prior Art
A d.c. high voltage power source is required for accelerating charged particles in, for example, apparatus utilizing a high energy electron beam, such as an electron microscope, an X-ray tube and an electron accelerator, as well as apparatus utilizing a high energy ion beam, such as an ion microscope, an ion microanalyzer, and an ion accelerator (hereinbelow, such devices shall be generically referred to as charged particle beam-applying apparatus).
In the charged particle beam-applying apparatus of this type, it is necessary to maintain a high stability in the acceleration energy of the charged particle beam employed. As the power source for accelerating the charged particles, therefore, a d.c. high voltage power source whose output voltage is highly stabilized need be used. To this end, a system is usually adopted in which a voltage divider for detecting the output voltage is connected across the high voltage output terminals of the power source and in parallel with a load. Then, the detected voltage at a voltage dividing point on the voltage divider is compared with a reference voltage, and an input of the power source is subjected to negative feedback control so as to reduce the measured difference between the dividing point voltage and the reference voltage to zero.
In the charged particle beam-applying apparatus of the specified type, it is also necessary to precisely know the current value of the charged particle beam which is utilized. It is, accordingly, required to measure and monitor the emission current from the charged particle source (namely, the load current of the d.c. high voltage power source) by some suitable method.
In principle, the load current of the high voltage power source can be determined by connecting an ammeter in series between the power source and the load and directly reading an indication thereof. However, where the output voltage value of the power source is so large as to be dangerous to the human body, it is undesirable from the viewpoint of both safety and actual installation to insert the ammeter on the high voltage side of the load.
In the charged particle beam-applying apparatus of the type described, the vacuum airtight casing thereof in which the charged particle beam (load current) flows is generally connected directly to ground. It is therefore impossible to connect the ammeter on the low voltage side of the load.
For these reasons, it has heretofore been common practice in the art to adopt a system in which, as illustrated in FIG. 1, the high voltage generating portion of the power source is connected to float above ground potential and the ammeter (or any other current measuring instrument as desired) is connected between the low voltage side output terminal and ground potential.
FIG. 1 shows an example of a prior art stabilized d.c. high voltage power source of this type. In the figure a d.c. high voltage generator 1 comprises a controlled input portion 2, an insulating transformer 3, and a voltage doubler circuit 4. The controlled input portion 2 may be provided in the form of an oscillator which is subjected to amplitude control in the well-known manner by an output of an error amplifier 7. The output of the controlled oscillator is boosted by the insulating transformer 3 and then converted into a d.c. high voltage by the voltage doubler circuit 4.
The negative high voltage V.sub.H which appears at the high voltage side output terminal 5 of the generator 1 is impressed on an electron source 9 of an electron beam-applying apparatus 8, such as a scanning electron microscope. As the result, an electron beam e is emitted from the electron source 9, and an emission current (load current) I.sub.e flows from the target of the device proportional to the beam current thereof. The low voltage side output terminal 6 of the high voltage generator 1 is grounded through an ammeter 10. Accordingly, the emission current I.sub.e flows to the ground potential via the ammeter 10.
On the other hand, voltage dividing resistances 12 and 13 forming a voltage divider for detecting the output voltage of the generator are connected in parallel with the load 8 between the output terminal 5 of the high voltage generator 1 and ground potential. A bias current I.sub.b flowing through the voltage dividing resistances 12 and 13 flows to ground via the ammeter 10 similar to the emission current I.sub.e.
In consequence of the flow of the bias current I.sub.b through the voltage dividing resistors 12 and 13, the following voltage appears at a voltage dividing point P: ##EQU1## where R.sub.1 and R.sub.2 denote the resistance values of the voltage dividing resistors 12 and 13, respectively. This voltage V.sub.p is applied to one input terminal of the error amplifier 7. To the other input terminal of the error amplifier 7, a reference voltage V.sub.s is applied by a reference voltage source 11. The error amplifier 7 amplifies the difference V.sub.p - V.sub.s between the respective input voltages and provides the amplified voltage as an output, by which the output voltage of the high voltage generator 1 is controlled and stabilized. That is, the output amplitude of the oscillator is controlled by the output of the error amplifier 7 in the controlled input portion 2, with the result that the output voltage V.sub.H of the high voltage generator 1 is subjected to negative feedback control so as to establish V.sub.p = V.sub.s. Thus, the output voltage V.sub.H of the high voltage generator is always stably maintained at a value of: ##EQU2## Needless to say, the value of the output voltage V.sub.H can be varied by changing the value V.sub.s of the reference source voltage.
As apparent from the above explanation, the emission current I.sub.e and the bias current I.sub.b superposedly flow through the ammeter 10, so that the indication of the ammeter 10 becomes I.sub.e + I.sub.b. The value of the emission current I.sub.e as the true load current cannot therefore be known merely by directly reading the indication of the ammeter 10. In order to know the value of the emission current I.sub.e in the circuit arrangement of FIG. 1, accordingly, it is required that a calibration curve of the bias current I.sub.b corresponding to the magnitude of the output voltage V.sub.H be prepared in advance and that the value of the bias current I.sub.b evaluated from the calibration curve be subtracted from the indication I.sub.e + I.sub.b of the ammeter 10. However, this method merely provides an approximation of the true value of I.sub.e, at best. Thus, the prior art circuit arrangement shown in FIG. 1 is undesirable, since it does not provide the possibility of directly reading the true value of the load current I.sub. e of the high voltage power source, in addition to the fact that the load current indication deduced therefrom has a low measurement precision.